Sulphur-containing polymers useful as stabilizing agents for rubber vulcanizates

ABSTRACT

Additives for rubber compositions, giving vulcanisates having improved retention of optimum physical properties, are polymeric trithiocarbonates having a structure comprising repeating units of the formula ##STR1## where R is an organic bridging group, e.g. an alkylene group.

This is a division of application Ser. No. 617,064, filed June 4, 1984,and issued Nov. 12, 1985 as U.S. Pat. No. 4,552,929.

This invention relates to rubber vulcanisates having improved physicalproperties.

The process of vulcanising diene rubbers by heating with sulphur and avulcanisation accelerator has been known for many years. By thisprocess, vulcanisates having certain physical properties, for instancetensile strength, resilience and fatigue resistance at a high level canbe obtained, but such vulcanisates tend not to have good ageingproperties. Apart from the addition of antioxidants which will retardoxidative heat ageing, other methods which have been proposed for makingvulcanisates having improved ageing properties include the use of lowerproportions of sulphur and increased proportions of accelerator relativeto those which would be employed for a conventional cure, and thepartial or complete replacement of sulphur by other cross-linkingagents.

Vulcanisates made using such alternative systems tend, however, to lackcertain of the merits of a sulphur-cured vulcanisate, and there istherefore a need for additives which will improve the ageing propertiesof the vulcanisate while retaining the merits of using sulphur as thevulcanising agent.

According to the present invention we have found that vulcanisateshaving improved properties can be obtained by adding polymerictrithiocarbonates in addition to sulphur and a vulcanisation acceleratorduring the compounding of diene rubbers. These materials have the effectof stabilising the properties of the vulcanisate if the temperature ofthe vulcanisate unavoidably remains high for a prolonged period aftercure, and during the service life of the vulcanisate. The inventionprovides a vulcanisable rubber composition comprising a diene rubber,sulphur and a vulcanisation accelerator, characterised in that thecomposition also contains a polymeric trithiocarbonate having astructure comprising repeating units of the formula ##STR2## where R isan organic bridging group.

A preferred class of polymeric trithiocarbonates useful in the presentinvention can be represented by the formula ##STR3## where R is anorganic bridging group, X represents a halogen atom or a group that hasreplaced the halogen atom, and n is the number of units in the polymermolecule.

The invention also includes a vulcanisate that has been obtained byheating a vulcanisable rubber composition of the invention at avulcanisation temperature.

In the above formulae, R can be, for example, a straight- orbranched-chain alkylene or alkenylene group, preferably one containingfrom 4 to 20 carbon atoms, and more preferably one containing 5 to 16carbon atoms. Examples of such groups are tetramethylene,pentamethylene, hexamethylene, octamethylene, nonamethylene,decamethylene, dodecamethylene, 3-methyl-1,5-pentylene and1,6-hex-2-enylene. As a variant, a divalent bridging group may be analkylene or alkenylene group having one or more aryl, for examplephenyl, substituents. An example of such a radical is2-phenyl-1,4-butylene.

In other instances, R has a structure comprising two or more alkyleneunits, pairs of such units being linked through an oxygen or sulphuratom, through a sequence of two or more sulphur atoms, through a group--COO--, or through an arylene or cycloalkylene radical. Representativeof such structures are those of the formulae

    --(CH.sub.2).sub.a --O--(CH.sub.2).sub.a --

    --(CH.sub.2).sub.a --O--(CH.sub.2).sub.a' --O--(CH.sub.2).sub.a --

    --(CH.sub.2).sub.b --A--(CH.sub.2).sub.b --

    --(CH.sub.2).sub.c --COO--(COO--(CH.sub.2).sub.a --

and

    --(CH.sub.2).sub.c --COO--Y--OOC--(CH.sub.2).sub.c --,

where each a' and each c independently represents an integer of from 1to 20, a represents an integer of from 2 to 20, each b independentlyrepresents an integer of from 1 to 10, A represents phenylene orcyclohexylene, and Y represents a group --(CH₂)_(c) -- or a group --(CH₂CH₂ O)_(d) CH₂ CH₂ -- where d represents an integer of from 1 to 5.Preferred values for a are from 2 to 8, preferred values for a' are from1 to 6, preferred values for b are 1 to 4, and preferred values for care from 1 to 18, more especially 1 to 12.

Where values of a, a', b, or c exceed 2, the polymethylene groups can bestraight chain or branched, but preferably the terminal carbon atom of Rwhich is attached to --S-- in formula (I) above is a primary carbonatom.

When X in formula (II) represents a halogen atom, this is usuallychlorine or bromine. Possible values of X when X represents a groupalkylthio, ammonium, alkylammonium and quaternary ammonium.

The polymeric trithiocarbonates of formula (II) above are new materials.

A general method for the preparation of trithiocarbonates of formula(II) where X represents a halogen atom comprises the reaction of adihalo compound of the formula X--R--X where R and X are as defined informula (I), with an alkali metal trithiocarbonate in the presence of aphase transfer catalyst, the alkali metal trithiocarbonate being presentin aqueous solution and the dihalo compound forming or being present ina water-immiscible organic phase.

Reaction temperatures of from 30° to 100° C. may be used, preferredtemperatures being those in the range 40° to 60° C.

In some instances, the product is an insoluble solid which can berecovered by filtration of the final reaction mixture. In otherinstances, the polymer is of lower molecular weight, n having an averagevalue of, for example, from 1.5 to 4, and forms a liquid organic phase.It is also possible to employ a low molecular weight polymer as thestarting material of formula X--R--X in the above process. Reaction withalkali metal trithiocarbonate gives a polymer of higher molecularweight, with n having an average value of, for example up to 25. Thus bysuitable selection of the proportions of reactants, reaction conditionsand the use, if necessary, of a multi-stage process, it is possible toobtain polymers having any desired degree of polymerisation.Purification and isolation of the polymeric trithiocarbonates can beeffected by conventional techniques.

Quarternary ammonium salts, for example tetrabutylammonium chloride, aresuitable phase transfer catalysts.

Polymers wherein X represents a group other than halogen can be preparedby reacting a polymer where X is halogen with a halogen-displacingreagent; for example an alkali metal alkoxide or an alkali metalmercaptan to give respectively alkoxy- or alkylthio-terminatedtrithiocarbonate polymers.

Polymeric trithiocarbonates useful in the present invention can also beprepared by the method described by Braun and Kiessel, Monatsh. Chem.96(2) 631-41 (1965), and by the method described in DE-A No.2 558 370.

The stabiliser materials referred to above are especially effective incompositions in which the rubber is cis-polyisoprene, either natural orsynthetic, and in blends containing at least 25% by weight ofcis-polyisoprene with other rubbers. Preferably the rubber, if a blend,contains at least 40% and more preferably at least 60% by weight ofcis-polyisoprene. Examples of other rubbers which may be blended withcis-polyisoprene include poly-1,3-butadiene, copolymers of 1,3-butadienewith other monomers, for example styrene, acrylonitrile, isobutylene,and methyl methacrylate, and ethylene-propylene-diene terpolymers.

The amount of stabiliser compound employed in the compositions of thepresent invention is usually from 0.5 to 5, for example from 1.5 to 5,parts by weight, and preferably from 2 to 4 parts by weight per 100parts by weight of rubber.

In the composition of the invention the essential vulcanising agent issulphur, but other vulcanising agents such as amine disulphides need notbe excluded. The amount of sulphur in the compositions is typically from2 to 3 parts by weight per 100 parts by weight of rubber, but lesser orlarger amounts, for example from 1 to 5 parts on the same basis, may beemployed.

In the composition of the invention, a single accelerator or a mixtureof accelerators can be employed. These include thioazole-basedaccelerators, for example 2-mercaptobenzothiazole, bis(2-benzothiazolyl)disulphide, benzothiazole-2-sulphenamides for instanceN-isopropyl-benzothiazole-2-sulphenamide,N-tert-butylbenzothiazole-2-sulphenamide,N-cyclohexylbenzothiazole-2-sulphenamide,N,N-diisopropyl-benzothiazole-2-sulphenamide,N,N-dicyclohexyl-benzothiazole-2-sulphenamide and2(morpholinothio)benzothiazole, thiocarbamylsulphenamides, for exampleN,N-dimethyl-N',N'-dicyclohexylthiocarbamylsulphenamide andN(morpholinothiocarbonylthio)morpholine. Mixtures of thiazole-basedaccelerators with diphenylguanidine can be used. Preferred acceleratorsare the benzothiazole-2-sulphenamides. In the compositions of theinvention, these are usually used in amounts of from 0.5 to 1.5 part byweight per 100 parts by weight of rubber.

The vulcanisate stabilisers used in this invention can be incorporatedinto rubber by conventional mixing procedures, for example by addingthem in a Banbury mixer or by adding them to the rubber on a mill.Ordinarily, with liquid or low melting solid vulcanisate stablisers, nospecial precautions are necessary for obtaining good dispersions.However, when using higher melting vulcanisate stabilisers it isrecommended that they be ground to a fine powder, preferably 70micrometer particle size or less to ensure adequate dispersion. Suchpowders may be treated to suppress dust, for example by the addition ofoil, or they can be mixed with a binder, for example a polymer latex,and formed into granules or pellets containing up to 5% weight ofbinder. They can also be formulated as predispersions in certain rubberypolymers, such as EPDM or ethylene-vinyl acetate rubber, whichpredispersions may contain, for example, from 15 to 50% by weight ofpolymer.

The rubber stocks may include reinforcing carbon blacks, pigments suchas titanium dioxide and silicon dioxide, metal oxide activators such aszinc oxide and magnesium oxide, stearic acid, hydrocarbon softeners andextender oils, amine, ether, and phenolic antioxidants, phenylenediamineantidegradants, and tackifiers. The stocks may also containprevulcanization inhibitors but in many stocks their use is unnecessary.

The invention is illustrated by the following Examples:

EXAMPLES 1-7

General method for the preparation of oligomeric trithiocarbonates. Anaqueous solution of sodium trithiocarbonate (50 ml, of concentration 1.4M/l) and an aqueous solution of tetrabutylammonium chloride (30 ml,prepared from 4 ml of 40% NBu₄ OH solution neutralised with hydrochloricacid) were well stirred in a flask fitted with a reflux condenser, towhich a dichloro compound (0.07 mole) was added. This mixture wasstirred at 50° C. for 9 hours, and the yellow oil was extracted intochloroform, washed with water and dried. Removal of the solvent in vacuoyielded the trithiocarbonate having the general structure--(R--S--CS--S)_(n) --R--.

Example 1

The dichloro compound was 4,4' dichlorodibutyl ether. The yield was 91%,and the NMR spectra indicated the above structure in which R=--(CH₂)₄--O--(CH₂)₄ -- and n=2.

Example 2

The dichloro compound was Cl(CH₂)₃ --CO--O (CH₂)₄ Cl. The yield was 97%,and the NMR spectrum of the product was consistent with the abovestructure, with R=--(CH₂)₃ --CO--0 (CH₂)₄ -- and n=1.5.

Example 3

The dichloro compound was ethylene glycol bis(2-chloroethyl)ether. Theyield was 93%, and the NMR spectrum was consistent with the abovegeneral structure, with R=--(CH₂)₂ --O--(CH₂)₂ --O--(CH₂)₂ -- and n=2.8.

Example 4

The dichloro compound was 2,2' dichlorodiethyl ether. The yield was 73%,and the NMR spectrum of the product was consistent with the abovestructure, with R=--(CH₂)₂ --O--(CH₂)₂ -- and n=2.5.

Example 5

The dichloro compound was 4.4' -dichlorodibutyl formal. The yield was96%, and the NMR spectrum of the product was consistent with the abovestructure, with R=--(CH₂)₄ --O--CH₂ --O(CH₂)₄ -- and n=2.

Example 6

The dichloro compound was 1,4-bis((chloromethyl)cyclohexane. The yieldwas 90%, and the NMR spectrum of the product was consistent with theabove structure, with R=--CH₂ --C₆ H₁₀ --CH₂ -- and n=2.2.

Example 7

The dichloro compound was an oligomeric trithiocarbonate obtained from4,4'-dichlorodibutyl ether with n about 3. The product was a polymerhaving --(CH₂)₄ --O--(CH₂)₄ -- as the group R in the above structure,and n=20.

Example 8

This Example describes the preparation of a polymeric trithiocarbonateusing a dibromo compound as starting material.

50 ml of a 1.4M aqueous solution of sodium trithiocarbonate and 30 ml ofan aqueous solution of tetrabutyl ammonium chloride (prepared asdescribed under Examples 1-7) were stirred in a flask fitted with areflux condenser. 0.07 Mole of 1,4-dibromobutane was added, and themixture was stirred at 50° C. for 9 hours.

The aqueous phase was then separated and chloroform was added to theorganic phase. The chloroform solution was filtered to recover a yellowsolid (8A). Evaporation of the filtrate gave a yellow paste (8B).

To evaluate the use of the polymeric trithiocarbonates according to thisinvention in natural rubber vulcanisates, a masterbatch having thefollowing composition was prepared:

    ______________________________________                                                            Parts by Weight                                           ______________________________________                                        Natural rubber        100                                                     Carbon Black          50                                                      Zinc Oxide            5                                                       Stearic Acid          2                                                       Processing Oil        3                                                       N--phenyl-N'--(1,3-dimethylbutyl)-                                                                  2                                                       p-phenylenediamine (Antidegradant)                                            ______________________________________                                    

Portions of the masterbatch were taken and mixed with sulphur,2(morpholinothio)benzothiazole and the stabiliser compound in theproportion 2.5, 0.7 and 3.0 parts by weight rrespectively per 100 partsby weight of rubber. A further portion of masterbatch to which onlysulphur and 2(morpholinothio)benzothiazole were added was used as acontrol.

The curing characteristics of the vulcanisable compositions thusobtained were determined at the curing temperatures shown in the Tablebelow by means ocf the Monsanto Oscillating Disc Rheometer, ISO3417-1977(E). From the Rheometer data, the time (t max.) required toreach maximum torque (maximum modulus) was recorded. Vulcanisates wereprepared by press curing at 141° C. for the time (t max.) indicated bythe Rheometer data to give maximum modulus. Other vulcanisates wereprepared at the same temperature but were held at this temperature for200 minutes (overcure). Both types of vulcanisate were subjected toconventional methods of physical testing. Resilience measurements werecarried out according to British Standard 903 part A8 (1963).

In the table below, the stabiliser compounds are identified by numbersaccording to the preceding Examples. Figures shown in parenthesis arethose obtained for the corresponding controls.

    __________________________________________________________________________                    % Retention of                                                                              % Retention of                                  Polymer 300% Modulus                                                                          300% Modulus                                                                          Resilience                                                                          Resilience on over-cure                         (Example No.)                                                                         (MPa).sup.(1)                                                                         on over-cure.sup.(2)                                                                  (1)   (2)                                             __________________________________________________________________________    1       16.1 (16.4)                                                                           98 (82) 68.9 (70.7)                                                                         90 (87)                                         3       17.5 (16.0)                                                                           94 (93) 67.6 (68.9)                                                                         91 (85)                                         4       17.8 (16.0)                                                                           107 (93)                                                                              64.9 (68.9)                                                                         96 (85)                                         5       15.4 (16.0)                                                                           99 (93) 68.3 (68.9)                                                                         90 (85)                                         7       18.7 (16.4)                                                                           93 (82) 65.3 (70.7)                                                                         90 (87)                                         8A      17.5 (16.4)                                                                           104 (82)                                                                              66.0 (70.7)                                                                         94 (87)                                         8B      17.5 (16.4)                                                                           100 (82)                                                                              61.8 (70.7)                                                                         95 (87)                                         __________________________________________________________________________     .sup.(1) Sample cured to t max.                                               .sup.(2) Result for sample cured for 200 minutes expressed as a percentag     of the result for sample cured to t max.                                 

These results show the value of the stabiliser compounds in alleviatingthe adverse effects of overcure.

We claim:
 1. A polymeric trithiocarbonate having the formula ##STR4## inwhich R has the structure

    --(CH.sub.2).sub.a --O--(CH.sub.2).sub.a --

or

    --(CH.sub.2).sub.a --O--CH.sub.2 --O--(CH.sub.2).sub.a --

where each a independently represents an integer from 2 to 8, and n hasan average value of from 1.5 to
 25. 2. The polymeric trithiocarbonate ofclaim 1 wherein R is --(CH₂)_(a) --O--(CH₂)_(a) -- and a is
 2. 3. Thepolymeric trithiocarbonate of claim 1 wherein R is --(CH₂)_(a)--O--(CH₂)_(a) -- and a is
 4. 4. The polymeric trithiocarbonate of claim1 wherein R is --(CH₂)_(a) --O--CH₂ --O--(CH₂)_(a) -- and a is
 2. 5. Thepolymeric trithiocarbonate of claim 1 wherein R is --(CH₂)_(a) --O--CH₂--O--(CH₂)_(a) -- and a is 4.